Although applicable to any given ultrasonic sensors and in various fields, the exemplary embodiments and/or exemplary methods of the present invention and the issues on which it is based are explained with regard to ultrasonic sensors which employ pulse propagation time methods, and the use of same in motor vehicles.
Ultrasonic sensors, in particular ultrasonic sensors which employ pulse propagation time methods, are used, for example, to perform automatic surroundings reconnaissance, for example in motor vehicles as parking assistance. A sensor emits an ultrasonic signal. The ultrasonic signal is reflected from an object or obstruction. The same sensor or a different sensor receives the reflected signal (referred to below as “received signal”), and from the received signal draws conclusions concerning the surroundings or obstructions. In the case of an ultrasonic sensor employing a pulse propagation time method, a signal burst having a predetermined pulse repetition interval, a defined burst length, and a defined signal frequency is transmitted as an ultrasonic signal for reconnaissance of the surroundings. The ultrasonic signal or the signal burst is generally referred to below as a “sonic transmitted signal.”
An active analogous duplex operation (for example, two-wire duplex operation) is often used for these types of ultrasonic sensors. In known methods, the capacitance of an ultrasonic sensor is charged on a line in the time-division multiplex. For three-wire designs a separate line may be provided for supplying power to the sensor. In such a case the duplex operation is carried out on a common line only between electrical transmission triggers and electrical received signals. An (electrical) transmission trigger which activates the ultrasonic sensor and thus the transmission of sonic transmitted signals (ultrasonic signals or signal bursts), i.e., acoustic transmitted signals of the ultrasonic sensor, is transmitted from a control unit, at the resonance frequency of a converter that is used (generally 40 kHz to 60 kHz), to the ultrasonic sensor. For cost reasons the frequency generation at the control unit is performed by the controller (μC) placed at that location. The (electrical) transmission trigger, for example in the form of a square wave signal, contains the transmission duration and the excitation frequency or resonance frequency.
If an object is present in the relevant surroundings, i.e., the surroundings to be searched by the ultrasonic sensor, the sonic or acoustic transmitted signal is reflected from this object. As a result of the acoustic reflection of the sonic or acoustic transmitted signal at the object the ultrasonic sensor receives an acoustic received signal. In the ultrasonic sensor the acoustic received signal is converted to an electrical signal by an ultrasonic converter, electrically amplified and AC-coupled to the same line, transmitted back to the control unit, and AC-decoupled in the control unit. In the control unit the decoupled received signal is subjected to a comparison, the comparison typically being performed on envelopes in a comparator with respect to a fixed threshold value or a simple characteristic curve, for example output by a μC. The envelope is implemented, for example, by noncoherent AM demodulation (for example, rectification and low-pass filtering).
The duplex operation of the transmission trigger and the back-transmission of the received signal are carried out in the same useful band at a carrier frequency which is similar or identical to the resonance frequency of the ultrasonic converter used (40 kHz to 60 kHz). The use of similar or even identical frequencies for the transmission trigger as well as for the analogous back-transmission of the electrical received signal requires a complicated locking of the transmission triggering in the ultrasonic sensor. The transmission triggering must be locked because the received signals electrically back-transmitted or reflected to the sensor may result in an unintended triggering of a transmission.